Gap tuning for surface micromachined structures in an epitaxial reactor

ABSTRACT

A method for adjusting with high precision the width of gaps between micromachined structures or devices in an epitaxial reactor environment. Providing a partially formed micromechanical device, comprising a substrate layer, a sacrificial layer including silicon dioxide deposited or grown on the substrate and etched to create desired holes and/or trenches through to the substrate layer, and a function layer deposited on the sacrificial layer and the exposed portions of the substrate layer and then etched to define micromechanical structures or devices therein. The etching process exposes the sacrificial layer underlying the removed function layer material. Cleaning residues from the surface of the device, then epitaxially depositing a layer of gap narrowing material selectively on the surfaces of the device. The selection of deposition surfaces determined by choice of materials and the temperature and pressure of the epitaxy carrier gas. The gap narrowing epitaxial deposition continues until a desired gap width is achieved, as determined by, for example, an optical detection arrangement. Following the gap narrowing step, the micromachined structures or devices may be released from their respective underlying sacrificial layer.

FIELD OF THE INVENTION

The present invention relates to manufacture of micromechanicalstructures, and relates in particular to a method for narrowing a gapbetween micromachined structures on a device during manufacture in anepitaxial reactor.

BACKGROUND INFORMATION

A method of depositing structural layers during manufacture ofsurface-micromachined devices sometimes involves the use of an epitaxialreactor. Epitaxy is a process for production of layers ofmonocrystalline layers of silicon over a single crystal substrate, andfor forming polycrystalline silicon layers over other substratematerials, for instance SiO₂ films on silicon substrates. Epitaxialreactors may be operated with precisely controlled temperature andenvironmental conditions to ensure uniform deposition and chemicalcomposition of the layer(s) being deposited on the target substrate. Inaddition to the precise control, use of an epitaxial reactor may permitbuild-up of layers on a substrate at significantly higher rates thantypically found with LPCVD (Low Pressure Chemical Vapor Deposition)systems.

U.S. Pat. No. 6,318,175 discusses an approach to using epitaxialdeposition to create a micromachined device such as a rotation sensor.

While the foregoing micromachining operations or similar processes mayprovide acceptable products for many applications, some applications mayrequire finer width gaps between the micromachined elements on thedevice than can be provided by this process. Some applications mayrequire, for instance, obtaining higher working capacitances and/orelectrostatic forces between micromachined structures. While etchingvery narrow trenches to obtain desired narrow gaps has been attempted,these methods may require slower etch rates, may be limited in aspectratio, and may be subject to limitations of the lithography and etchingprocess. Similarly, germanium has been applied to produce narrow gaps,however, this process may have process compatibility limitations.

Accordingly, there is a need for a process for manufacturing deviceswhich provides product with inter-element gaps that may be preciselydefined or “tuned” to meet the device design objectives, while stillmaintaining satisfactory production rates.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a methodfor precisely controlling the gap between micromechanical elements on adevice, or “gap tuning,” begins with a partially formed micromechanicaldevice, which may comprise a substrate layer of, for examplemonocrystalline silicon or a SiGe mixture. A sacrificial layer of, forexample, SiO₂ may be deposited on the substrate layer. A functionallayer of, for example, epitaxially deposited silicon, may be etchedafter application to define micromechanical structures or devicesthereon.

Once the elements of the micromechanical structure or device have beendefined in the function layer and the sacrificial layer, in situcleaning of the device within the epitaxial reactor may be performed.The cleaning may be performed, for example, with hydrogen (H₂) to removesurface oxides, and/or with hydrochloric acid (HCI) to remove siliconresidues and surface imperfections resulting from the trench etchingprocess. Following the cleaning step, gap tuning may be performed byselectively depositing an epitaxially-grown layer of silicon on thesurface of the partially completed device, and in particular on thesides of the previously etched trenches defining the micromechanicalelements in the function layer. As the gap-tuning layer is deposited,the gap width may be monitored, for example with an optical end-pointdetection system. The gap tuning deposition may be halted when theinter-element gap has been narrowed to the desired extent.

The precision control of the width of the gaps between themicromechanical elements on a device in the foregoing manner may provideseveral advantages including: ready compatibility with an epitaxialenvironment and standard epitaxy equipment; high production rates due tothe high layer deposition rates that may be achieved in an epitaxialreactor; and ready adaptability to use of different materials to bedeposited on the micromachined device, including monocrystallinesilicon, polycrystalline silicon, a SiGe mixture, pure germanium, orsilicon carbide. Furthermore, the deposited layers may be in situ doped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a through 1 f show cross-section and plan views of various stagesof preparation of an exemplary micromachined device.

FIG. 2a illustrates an exemplary embodiment of the present inventionshowing the addition of material to the exemplary micromachined device.

FIG. 2b illustrates an exemplary method for detecting the desiredinter-element gap width.

FIG. 3 shows the exemplary embodiment of FIGS. 2a and 2 b undergoingsputtering to remove undesired epitaxially deposited material.

FIG. 4 is a flowchart illustrating steps for achieving the desiredinter-element gap width in accordance with an exemplary embodiment ofthe present invention.

FIG. 5 shows a cross-section of an exemplary device having multiplelayers.

DETAILED DESCRIPTION

According to an exemplary embodiment of the present invention, a methodfor gap tuning a micromachined structure or device is provided. As shownin the cross-section view in FIG. 1a, the partially formed device isbased on a substrate layer 1 of, for example, substrate silicon, uponwhich a sacrificial layer 2 of, for example, SiO₂ is deposited in an LowTemperature Oxide (LTO) process or thermally grown. FIG. 1b shows across-section view of the substrate/sacrificial layer combination ofFIG. 1a after a pattern of holes or open areas 3 have been formed insacrificial layer 2 using etching techniques, for example by applicationof a photo-sensitive material over the sacrificial layer, applying amask with the desired etching pattern over the photo-sensitive material,exposing the masked surface to light, and then applying etchants toremove the exposed portions of the photo-sensitive material then thesacrificial SiO₂ underneath the exposed portions. FIG. 1c shows a planview of the partially formed device of FIG. 1b showing holes defined bythe etching process through sacrificial layer 2. The cross-section viewin FIG. 1b is taken through the line IB—IB of FIG. 1c.

The partially formed device may then receive an epitaxially depositedfunction layer 4 of, for example, silicon, as shown in the cross-sectionview FIG. 1d. The portions 5 of the function layer 4 formed on the SiO₂have a polycrystalline structure, while the portions 6 of the functionlayer 4 formed on the silicon substrate layer 1 have a monocrystallinestructure. As shown in FIG. 1e, function layer 4 may then be etched inthe previously described manner to define the micromechanical structuresor devices in function layer 4. This etching may include deep, narrowtrenches 7 etched into the exposed portions of the photo-sensitivematerial and the underlying polycrystalline silicon of function layer 4.The trench etching processes may penetrate function layer 4 possibly upto the SiO₂ of sacrificial layer 2.

FIG. 1f is a plan view of the partially formed device showingmicromechanical elements 8 defined by the etched trenches. Thecross-section views in FIG. 1d and FIG. 1e are both taken through theline IE—IE, which also corresponds to line IB—IB of FIG. 1c. Deflectionbeam portions 9 of micromechanical elements 8 are shown in FIG. 1eextending from base portions 10 of micromechanical elements 8. Baseportions 10 may be firmly affixed to the silicon substrate 1, whiledeflection beam portions 9 may rest upon, and may therefore berestrained by, underlying layer 11 of SiO₂ of sacrificial layer 2. Thislayer of sacrificial material needs to be removed to free deflectionbeams 9 to deflect from their rest position during operation of themicromechanical device. In an exemplary embodiment, after deflectionbeams 9 are freed they are free to deflect in a direction perpendicularto their longitudinal axes. This movement results in a change in gaps 7between the beams, which may in turn cause detectable changes in thecapacitance between the beams.

With the partially formed device manufactured to the point of having thetrenches 7 etched through functional layer 4, the process of alteringthe width of the gaps between micromechanical elements 8 to preciselythe desired gap may proceed. If a SiO₂ hard mask is present as a resultof prior device fabrication steps, it may remain in place on the device,as it may be removed during the subsequent micromechanical elementrelease steps, and it may minimize the deposition of gap-fillingmaterial on the upper surfaces of the device, where it may not be needed

Following the cleaning steps, the gap tuning process may continue withdeposition of an epitaxially grown layer of material such asmonocrystalline silicon, polycrystalline silicon, germanium, and/or SiGeon the partially formed device. The preferred choice of material to beepitaxially deposited may be determined by the nature of the material tobe coated and the geometry of the gap to be narrowed. For example, thefunction layer may be formed from monocrystalline silicon. The preferredgap narrowing material may be carried in the epitaxial environment by anH₂ flow. The environmental parameters of the H₂ flow, includingtemperature, pressure, and the chemical composition of the gap narrowingmaterial, may be varied to achieve selective deposition of the gapnarrowing material on the various regions of the device. For example, ifsilicon is to be deposited to narrow the trenches 7, one of silane,dichlorosilane, or trichlorosilane may be provided. HCI may be includedto cause the silicon deposition to be more selective, i.e., to cause the

silicon to deposit on the surfaces of the micromechanical elements 8formed in function layer 4, but not on the exposed surfaces of the SiO₂of sacrificial layer 2.

An illustration of this selective deposition is shown in FIG. 2a. FIG.2a is a partial cross-section of the device taken along the line IIA—IIAshown in FIG. 1f. FIG. 2a depicts the arrangement of micromechanicalelements 8 separated from one another and their respective adjacentsections of function layer 4 by trenches 7. Prior to initiating the gapnarrowing process, the trenches 7 are wider than desired, as indicatedby the width shown by arrow 12. Once the deposition of the gap narrowingmaterial is initiated, the material may begin to build-up a depositedlayer 13 on the upper surface of function layer 4 and micromechanicalelements 8 and on the vertical sides of trenches 7. Deposited layer 13may increase in thickness as the epitaxial deposition process continues,until a desired thickness is reached, corresponding to a desiredinter-element gap 14. In this exemplary embodiment, because sacrificiallayer 2 is composed of SiO₂ and process parameters are controlled toprovide selective deposition, no gap narrowing material is deposited attrench bottoms 15. In other words, the process parameters are adjustedto cause selective deposition in which the gap narrowing materialdeposits on the function layer, but not on the sacrificial (SiO₂) layer.This ensures that the portions of sacrificial layer 11 underlyingmicromechanical elements 8 remain exposed to facilitate their possiblesubsequent removal to free micromechanical elements 8.

The termination of epitaxial deposition of gap narrowing material may becontrolled in a variety of ways. For example, the gap narrowingdeposition process may proceed in a step-wise fashion, with periodichalts to permit close examination of the extent of gap narrowing thusfar achieved. Preferably, the gap narrowing material deposition may becompleted in one step, with periodic or continuous monitoring of gapwidth occurring throughout the formation of deposition layer 13. Suchin-process gap width monitoring and deposition termination control maybe performed by an optical end-point detection system. An optical systemsuitable for use with the present invention may detect gap thickness by,for example, looking at an interference pattern of reflected light fromthe surface of the device. Alternatively, as in the exemplary embodimentillustrated in FIG. 2b, an optical system may compare the reflectedlight from higher surfaces 16 and reflected light from lower surfaces 17at the bottom of trenches 7.

An illustration of non-selective deposition in accord with an exemplaryembodiment of the present invention is shown in FIG. 3. FIG. 3 is apartial cross-section of the device taken along the line IIA—IIA shownin FIG. 1f. FIG. 3 depicts the arrangement of micromechanical elements 8separated from one another and their respective adjacent sections offunction layer 4 by trenches 7. The deposition of the gap narrowingmaterial may cause build-up of deposited layer 13 on the upper surfaceof function layer 4 and micromechanical elements 8 and on the verticalsides of trenches 7. In this exemplary embodiment, the processparameters are controlled to provide non-selective deposition, andtherefore the gap narrowing material deposits everywhere. Therefore,since sacrificial layer 2 is composed of SiO₂ and the process parametersare controlled to provide nonselective deposition, the gap narrowingmaterial may be deposited at trench bottoms 15. Furthermore, FIG. 3shows the results of a highly conformal deposition, and therefore thedeposition of the gap narrowing material is uniform between the top(portions 19 a) and trench bottoms 15 (portions 19 b). In analternative, non-conformal deposition, the deposition rate may be higherat the top (portions 19 a) than trench bottoms 15 (portions 19 b). Amethod for removing portions 19 b of deposited layer 13 on the bottom oftrenches 7 and portions 19 a on top of micromechanical elements 8, whileleaving portions 20 of deposited layer 13 arranged on the verticalsidewalls of micromechanical elements 8, may be desirable. FIG. 3 showsan exemplary method for removing portions 19 a and 19 b of depositedlayer 13 by sputtering. Sputtering may involve ionizing particles, forexample Argon, in a plasma region above the surface of the device, thenaccelerating the ions in an electrostatic field in the direction ofarrows 21. The ions may then impact the surface of the device, therebyimparting mechanical energy to the surface of the device, therebydislodging particles from the surface. This sputtering may thereforehave the effect of removing material in a mostly uniform manner fromsurfaces perpendicular to arrows 21. Therefore, the vertical sidewallsof micromechanical elements 8 may remain relatively untouched by theions, and may therefore retain some or all of portion 20 of depositedlayer 13.

Following completion of the gap narrowing process, micromechanicalelements 8 may be released from their underlying columns of sacrificiallayer material using any method. Metals may be added to the device afterthe gap tuning and prior to micromechanical element release processsteps.

In an alternative exemplary embodiment an SOI (Silicon On Insulator)wafer may be utilized. The insulator layer of the SOI wafer may form asacrificial layer and a top silicon layer of the SOI wafer may form thetop layer.

FIG. 4 is a flowchart showing a detailed implementation of an exemplarymethod for tuning the gap between micromechanical elements and releasingthe micromechanical elements from the underlying sacrificial layer. Theprocess method starts at step 100 with a device into which trenches havebeen etched to define an element of a micromechanical structure ordevice. In step 110, which is an optional step in the method, residualmaterials from the trench etching process may be removed. Step 110 isfollowed by step 120, placing the device in an epitaxial reactor. Step120 is followed by step 130, removing residual oxides from the surfaceof the micromechanical device that remain following the trenchingprocess by exposing the surface of the device to H₂ gas and/or removingsilicon residues remaining following the trenching process by exposingthe surface of the device to HCI. Alternatively, step 130 may beskipped, and the flow may proceed directly from step 120 to step 140.Step 130 is followed by step 140, epitaxially depositing a gap narrowingmaterial on selected surfaces of the device until a desiredinter-element gap width is achieved. After step 140, the device may beremoved from the epi-reactor. Step 150 removes the exposed sacrificiallayer material by flowing HF gas over the device. Step 150 is alsooptional depending on the desired device, and therefore the flow mayproceed directly from step 140 to step 160. Step 160 marks the end ofthe micromechanical element gap tuning and release portion of amicromachined device manufacturing process.

FIG. 5 shows handle wafer 51, which may be a silicon wafer, with devicelayer 52 arranged above handle wafer 51 and defining a cavity which isfilled by sacrificial material 54. Encapsulation layer 53 is arranged ontop of device layer 52 and includes vents 55 which access sacrificialmaterial 54. Vents 55 may be gap tuned in the manner described above bydepositing gap-tuning layer 57. In a subsequent process step,sacrificial material 54 may be etched or release using any appropriatetechnique. In this manner, an exemplary device having multiple functionlayers and multiple sacrificial layers may be constructed, which mayinclude vents 55 tuned by gap-tuning layer 57 as well as device 56. Inan alternative exemplary embodiment, more sacrificial layers and morefunction layers may be arranged above encapsulation layer 53.

While the present invention has been described in connection with theforegoing representative embodiment, it should be readily apparent tothose of ordinary skill in the art that the representative embodiment isexemplary in nature and is not to be construed as limiting the scope ofprotection for the invention as set forth in the appended claims.

What is claimed is:
 1. A method for tuning a gap between a plurality offaces of at least one micromechanical element on a device, comprising:etching an outline of the at least one micromechanical element in a toplayer of the device, the outline defining at least two opposing faces ofthe plurality of faces of the at least one micromechanical element; anddepositing in an epitaxial reactor a gap-narrowing layer on the at leasttwo opposing faces of the plurality of faces; wherein a gap between theat least two opposing faces of the plurality of faces is narrowed by thegap-narrowing layer.
 2. The method of claim 1, wherein the gap-narrowinglayer includes a silicon layer.
 3. The method of claim 1, wherein thegap-narrowing layer includes a germanium layer.
 4. The method of claim1, wherein the gap-narrowing layer includes a silicon/germanium layer.5. The method of claim 1, wherein the device includes: a substratelayer; a sacrificial layer deposited on at least a first portion of thesubstrate layer; and a function layer deposited on at least a secondportion of the sacrificial layer to form the top layer.
 6. The method ofclaim 5, wherein: the sacrificial layer includes silicon dioxide; andduring the deposition of the gap narrowing layer, deposition on thesacrificial layer is selectively avoided by adjusting at least one of atemperature, a pressure, and a gas composition of the epitaxial reactor.7. The method of claim 6, wherein the gas composition of the epitaxialreactor includes a compound of at least one of bromine, chlorine,fluorine and hydrogen.
 8. The method as recited in claim 5, furthercomprising at least one of: providing a further sacrificial layer; andproviding a further function layer.
 9. The method as recited in claim 8,wherein the at least one of the providing of the further sacrificiallayer and the providing of the further function layer is performedbefore the etching of the outline operation.
 10. The method as recitedin claim 8, further comprising etching the further sacrificial layer.11. The method of claim 1, wherein the device includes an SOI wafer, aninsulator layer of the SOI wafer forming a sacrificial layer and a topsilicon layer of the SOI wafer forming the top layer.
 12. The method ofclaim 1, wherein the depositing operation further comprises: entrainingone of silane, dichlorosilane and trichlorosilane in an H₂ flow; andpassing the H₂ flow over the device.
 13. The method of claim 1, furthercomprising: detecting a remaining gap width; and terminating thedeposition of the gap narrowing layer when the remaining gap width isabout equal to a desired gap width.
 14. The method of claim 13, whereinthe detecting operation includes detecting the remaining gap width withan optical detector.
 15. The method of claim 14, wherein the opticaldetector detects an interference pattern from light refracted from thedevice.
 16. The method of claim 14, wherein: the optical detectordetects a first light reflection reflected from an upper surface of thedevice and a second light reflection reflected from a lower surface ofthe device; the first light reflection includes at least one of a firstlight intensity and a first light phase; the second light reflectionincludes at least one of a second light intensity and a second lightphase; and a ratio between the first light reflection and the secondlight reflection is determined.
 17. The method of claim 1, wherein thedeposition of the gap narrowing layer includes a selective depositionprocess.
 18. The method of claim 1, wherein the deposition of the gapnarrowing layer includes a conformal deposition process.